The invention relates to an image processing method comprising steps of acquiring a sequence of overlapping images representing an elongated scene and steps of merging said images into an overview image.
The invention also relates to an image processing system to perform said method. The invention further relates to an X-ray apparatus comprising an image processing system for performing said method.
The invention finds its application in the field of medical apparatus.
In medical X-ray imaging it is necessary to form images of large areas, notably X-ray images of blood vessels in the limbs in the field of peripheral X-ray angiography, or spinal column images, or complete region radiography of an arm or a leg of a patient. Using a standard X-ray apparatus at the present time, it is difficult to form images of such large regions in one exposure. Generally, a sequence of successive X-ray images of zones of the region to be examined is formed in such a manner as to cover the entire region.
Such an image processing method is already known from European Patent Application EP 0861556, published under the PCT reference number WO98/08338. This document discloses a method of forming an overview image of an elongated scene which is too long to be imaged by acquiring only a single standard image. The overview image is formed by merging successive overlapping images of an image sequence of the elongated scene while using steps of combining image pairs on the basis of their individual shift. The images may be X-ray images formed by irradiating a patient by means of an X-ray beam in different positions.
According to the cited document, the elongated scene comprises a first and a second kind of object which are constituted by a ruler and by elongated anatomical structures respectively. The two kinds of objects being situated at different distances from the X-ray source, they are imaged in the overlapping images with parallax artefacts. To solve this problem, the known method comprises steps of separately processing first and second portions of the image, representing the ruler and the anatomical structures respectively. First and second shift values are derived from an image pair for the first and second image portions which are further combined independently by application of a correlation to their pixel values. The first resultant combined portion, representing the ruler is then either compressed or expanded, taking into account the ratio of the two shift values, in order to provide a parallax corrected combined first portion. A combined image of the image pair is further constructed by sewing the two resultant combined portions together. The overview image is further constructed by combining all the combined images obtained from the image pairs.
In the known method, the first and the second shifts are assumed to be substantially constant within said first and second image portions over the whole sequence, but this assumption is not verified. Parallax occurs because individual parts of the elongated scene are situated at different distances from the X-ray source and are viewed from different positions of the X-ray source either for different objects in the same image or for the same object represented in different overlapping images.
A first problem in the known method is that there are still parallax artifacts in the overview image. This may result in that, in the overview image, reconstructed bones show hachured walls or doubled joints, reconstructed vessels seem doubled or partly disappear and the ruler shows blurred or split graduations.
Another problem of the known method is that it may not be fully automated because the first and second portions of images representing the ruler and the anatomical structures must be selected by visual inspection of the images.
Another problem of the known method is that user interaction is further necessary for determining structures of interest of said first and second portions so as to evaluate the correlation laws and shift values.
Another problem of the known method lies in finding a background zone for operating a boundary sewing of the first and second combined portions side by side, after they have first been processed separately.
It is an object of the present invention to provide a method, a system and an apparatus for constructing an overview image from a sequence of images, in which parallax effects are counteracted better than in the known method.
This object is achieved by an image processing method comprising steps of acquiring a sequence of overlapping images representing an elongated scene, said images being obtained at different positions along a translation direction, and steps of merging said images into an overview image, including determining regularized displacement fields of overlapping regions of interest (ROI) and sections (Bj) of said regions in image pair and merging said section based on dimensions calculated from the regularized region displacement field. A system of the invention comprises an arrangement of calculation means for carrying out the methods of the invention. An apparatus of the invention comprises acquisition means for acquiring examination images of a patient, translation means for acquiring translation of consecutive overlapping images of an elongated part of the patient, and an image processing system comprising an arrangement of calculation means for carrying of the methods of the invention claim 16.
An advantage of this method is that it improves image quality by reducing parallax artifacts without increasing the acquisition sequence duration and hence the X-ray dose for the patient in the case where the image frames are formed using an X-ray apparatus.
Another advantage of this method is that it does not need user interaction to be performed. It may be performed completely automatically.
Another advantage of this method is that it does not rely on structures of interest to construct the composite image. The image frames may comprise several kinds of objects, such as anatomical objects and rulers, or only one kind of objects such as purely anatomical objects. This method is carried out independently of the nature of objects in the image frames.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.